A phenomenon known as “sleep inertia” often manifests itself after a person wakes up. This is a common problem, especially if the person did not get enough sleep, or was awoken abruptly, or was awoken from a deep sleep. A person with sleep inertia might demonstrate all the outward physical signs of being awake, and yet not be cognitively awake. Symptoms of sleep inertia include impairment of performance and reaction time when thinking or using motor skills, reduction in memory, and impairment of decision-making ability. Sleep inertia causes many people to function poorly without ritual intake of caffeine, which may alleviate the condition. Sleep inertia can also be dangerous for people who drive in the early morning hours shortly after waking.
Sleep patterns vary from person to person, but everyone attains several different stages of sleep each night. These stages are generally broken down into NREM sleep, or Non-Rapid Eye Movement sleep and REM sleep, or Rapid Eye Movement sleep. NREM is a deeper sleep than REM sleep in certain respects. The deepest stages of NREM sleep (called stages 3 and stage 4 sleep) usually occur during the hours after going to sleep, instead of in the morning hours before waking up. It is very advantageous to wake up gradually instead of abruptly, and to wake up during or shortly after a period of REM sleep instead of during deeper NREM sleep, in order to reduce sleep inertia. In general, it can be much more pleasant to wake up in this natural manner.
Waking up is especially problematic for travelers between time zones, who are susceptible to jet lag and the like. In contrast, people who get a good night's sleep at home are not likely to be in a deep NREM sleep when they wake, but even for them it can be advantageous to wake up slowly and in REM sleep or near a transition from REM to NREM sleep, instead of waking abruptly or from an NREM sleep that is not near a transition from REM sleep.
It is known to use telephones for purposes of waking people up. For example, many hotels offer wake-up calls to their customers, which eliminates the need for a traveler to take along an alarm clock. However, these typical wake-up systems arouse a person at a specific requested time, instead of when the user is in an appropriate sleep stage. Even if sleep sensors were made available to hotel guests, the hotel would be vulnerable to theft of those sensors. Moreover, a normal hotel visitor would dislike attaching unfamiliar sensors to his or her body at night, including sensors which would have been used by another guest the previous night. No solution has yet been devised to combine the convenience of telephonic wake-up with the benefits of waking up in an appropriate sleep stage.
There are various ways to use sensors in order to detect REM sleep. During REM sleep, the heart rate and cardiac output increase compared to NREM sleep, and therefore heart rate sensing can be used to estimate when REM sleep is occurring. Also, during REM sleep, the physiological functions regulating body temperature are inactive, and the body temperature may thus rise or fall, depending on the ambient temperature. Sweating and shivering is practically nonexistent but may occur if the ambient temperature is extreme. Also, body movements are inhibited during REM sleep.
Another indicator of REM sleep is blood vessel constriction (vasoconstriction) that can be measured, for example, in the hands and feet. Blood flow in the arterioles and capillaries of the extremities decreases sharply during REM sleep, due to vasoconstriction, and this can be measured, for example, using Peripheral Arterial Tone (PAT) measurement, as described by Goor et al. (U.S. Pat. No. 6,322,515). An algorithm presented by Lavie et al. in SLEEP, Vol. 26, Abstract Supplement (2003), p. A385–A386 may be used to detect REM sleep with the aid of PAT measurement
Another way to detect REM is by directly sensing eye movement, as disclosed by Laberge et al. (U.S. Pat. No. 5,507,716). Laberge discloses a comfortably worn face mask covering face portions, and a headband supporting the face mask about the person's head. The face mask, in turn, supports components of this equipment, which sense a person's eyelid movements during sleep. The sensing components supported on the face mask utilize a low level infrared emitter positioned on the face mask to direct infrared light to the eyelid of a sleeping person, and also utilize a low level infrared detector to receive the reflections of this infrared light from the surface of the eyelid of the sleeping person. The Laberge invention is directed at enhancing dreams, and therefore includes other features in addition to the REM detecting components. It is noted that many people already wear an eye mask (also called a sleep mask or blindfold) when going to sleep, in order to keep out light, and those existing eye masks may also keep out sound and/or provide soothing sound.
Alarm clocks correlated to REM have been devised, but the existing art does not address certain problems. The main problem with the prior art solutions is that they are inconvenient to take along when travelling. However, sleep deprivation and disturbances of the circadian rhythm (i.e., disturbances of a person's internal 24-hour biological clock) are very common when travelling, due to jet lag, fragmentary sleep, and long travel times. The need for a natural alarm clock is thus even greater when travelling. The problem of the prior art solutions is thus their practical immovability. They can of course be moved but they are rather large and heavy to take along when travelling. Most prior art solutions include a central unit, and the natural alarm clock is thus comparably bulky, especially for people preferring to travel light (e.g., with cabin baggage only).
Alarm clocks correlated to REM are referred to here as natural alarm clocks. The existing natural alarm clocks fall into several categories. Natural alarm clocks based on electroencephalography (EEG) measurements include Lidow (U.S. Pat. No. 4,228,806), Krischenowski (DE19642316), Cohen et al. (U.S. Pat. No. 4,776,345), Schroeder (EP0496196), Choi (KR2001095796), and Aizawa (JP9264977). Natural alarm clocks based on heart rate include Koyama et al. (U.S. Pat. No. 5,101,831), Bae et al (KR321532), Knutzen et al. (DE4209336), and Westerfeld (DE19916944). Natural alarm clocks based on temperature or skin resistance measurement include Youdenko (US2002/0080035), Halyak (U.S. Pat. No. 5,928,133), and Matsuura (JP2001017550). Natural alarm clocks based on accelerometers include Barron et al. (EP1163877), Watanabe et al. (JP2002372593), and Yanai et al. (JP2000316832). Natural alarm clocks based on Static Charge Sensitive Beds (SCSB) include Pellet (FR2665080). Natural alarm clocks based on muscle signals detected using electromyography (EMG) include Boucheron (FR2634913). Other natural alarm clocks include Masuda et al. (EP1059575), Boucheron (FR2597995), Yoshida (JP2001242268), Miura (JP2001116866), and Beno (DE4303933).
The cause of sleep inertia is not totally clear. Heart and respiratory rate decrease during NREM sleep, the activity of the sympathetic nervous system is low, distal blood vessels are relatively dilated, and blood pressure is decreased. These facts imply that the cerebral blood flow is relatively low. The decreased respiratory rate may also imply that the oxygen saturation of the blood is relatively low and/or the carbon dioxide saturation relatively high, so the oxygen levels of the brain may be relatively low and the carbon dioxide levels correspondingly high. The brain is also in a passive state during NREM sleep. In contrast, during REM sleep, heart and respiratory rate and blood pressure are variable and increased compared to NREM sleep, and vasoconstriction occurs as a result of increased sympathetic nervous activity. These facts seem to suggest that cerebral blood flow is relatively high during REM sleep, compared to NREM sleep. The increased respiratory rate seems to suggest that the oxygen saturation level of the blood is relatively high during REM and the carbon dioxide saturation level of the blood is correspondingly relatively low compared to NREM sleep. It thus seems probable that the oxygen levels of the brain would be higher at the end of a REM sleep stage than during the end of a longer NREM stage or at the beginning of a REM stage. According to a similar reasoning, the carbon dioxide levels of the brain would be relatively low at the end of a REM sleep stage compared to the end of an NREM sleep stage or the beginning of a REM sleep stage. In addition, the brain is in a highly active state during REM sleep. These findings seem to suggest that the operational premises of the brain are better at the end of an REM sleep stage than at the end of a longer period of NREM sleep or at the beginning of a REM sleep stage.
Kräuchi et al. suggest a correlation between distal vasoconstriction and sleep inertia in their abstract in SLEEP, Vol. 26, Abstract Supplement, 2003 p. A56–A57. This seems to suggest that it would be beneficial to use a vasoconstriction related measurement as a sleep characteristic measurement.
As previously discussed, some examples of sleep characteristics are: heart rate information; heart rate variability information; ECG (electrocardiography) signals; EEG (electroencephalography) signals; respiratory rate information; respiratory rate variability information; vasoconstriction or vasodilatation measurement signals such as PAT (peripheral arterial tone), PPG (photoplethysmography), PTT (pulse transit time), or IPG (impedance plethysmography) signals, or variation information regarding PAT, PPG, PTT and/or IPG; body temperature and/or distal temperature information; blood pressure information; and actigraphy, accelerometer, or movement sensor information, with or without sleep stage or depth information.